The stroke of a proportional solenoid consists of a proportional stroke portion and a nonproportional stroke portion. In the proportional stroke portion, the solenoid can generate a magnetic force substantially proportional to current applied to a coil, and the position of an armature can be controlled by means of the magnetic force. In the nonproportional stroke portion, such control cannot be carried out. Usually, only the proportional stroke portion is used.
A conventional proportional solenoid generally has a length of the above-mentioned proportional stroke of about 2 or 3 mm as well as a large volume. Accordingly, the proportional solenoid has a length 10 times or more the length of a proportional range (the above-mentioned proportional stroke), resulting in an increase in manufacturing cost.
Patent Document 1 shows an example of a conventional method for extending the proportional range of a stroke.
FIG. 11 is an explanatory view of a proportional solenoid disclosed in Patent Document 1.
FIG. 11(a) is a sectional view showing essential portions of the proportional solenoid, excluding a coil wound on a bobbin, and a yoke which forms a magnetic path.
A stationary core 101 has an annular projection 103 at its end located on a side toward an armature 102. The annular projection 103 has a frusto-conical surface 103a extending toward a center axis, and a central recess 108 formed in the annular projection 103.
The armature 102 has an annular projection 104 at its end located on a side toward the stationary core 101, the annular projection 104 being received in the recess 108. The annular projection 104 has a conical recess 104b having a conical surface 104a extending outward from the center axis.
Such a conical section in which the annular projection 103 of the stationary core 101 having the inward extending frusto-conical surface 103a and the annular projection 104 of the armature 102 having the outward extending conical surface 104a move in a mutually overlapping manner exhibits a force vs. stroke characteristic having a linear segment as shown in FIG. 11(b).
In FIG. 11(b), a characteristic 111 between line B and line C is a substantially constant force vs. stroke characteristic, and a characteristic 112 between line C and line E shows an overtravel stroke. The overtravel stroke is used when an additional stroke gap is required beyond the characteristic 111. A broken-line characteristic 110 between line A and line B is a generally undesirable force-rising characteristic.
In order to eliminate the force-rising characteristic 110, a non-magnetic brass shim 105 having a thickness equal to the length between line A and line B is disposed as a stopper at a portion of stroke between line B and line A. Specifically, the shim 105 is secured to a push rod 106 so as to be disposed between the surface of the recess 108 of the stationary core 101 and the end face of the projection 104 of the armature 102.
Patent Document 1: Japanese Patent Application Laid-Open